1. Field of the Invention
This invention relates to a tracking error signal detecting device which is applicable to an optical head for reproducing, with light, information recorded on an information carrying member.
2. Description of the Prior Art
The optical head as mentioned above generally includes a light source such as a laser or the like, an optical system comprising a collimator lens, objective lens or the like, and a light receiving element such as a PIN photodiodes or the like. The light beam emerging from the light source is converged on an information carrying member such as a disc and is optically modulated by the information recorded on the information carrying member, thereafter the modulated light is photoelectrically converted by the light receiving element whereby the recorded information is converted into electrical signals. Such optical head is applicable to an optical video disc, optical digital audio disc, optical information file and the like. The recorded information usually comprises fine or minute patterns formed concentrically or spirally on a rotatable disc-like information carrying member disc). These patterns are defined by surface unevenness, partially formed holes, variation in the reflection factor, difference in the direction of magnetization or the like.
In any application of the optical head, the patterns must be minute in order to improve the recording density. Therefore, the reproduction of such recorded information on the information carrying member requires that the converging beam precisely traces the track comprising minute pattern array. In a case of the aforesaid disc, for example, the track will displace in the radial direction of the disc with the rotation thereof due to any eccentricity or the like. This requires the provision of means for moving the converging beam or the disc to trace the track even if it is concentric, and, in turn requires detection of a tracking error signal which represents deviation of the converging beam from the track.
There are known several tracking error signal detecting device such as follows:
(1) Tracking Error Signal Detecting Device of Three-Beam System
FIG. 1 of the accompanying drawings shows an example of this device in which the light beam emerging from a light source 1 such as a semiconductor laser is collimated by a collimator lens 2 and the collimated beam is split by a diffration grating 3 into three light beams. These split light beams after transmission through a half-mirror 4 are converged by an objective lens 5 on an information carrying member 6 such as a disc. At this time, as shown in FIG. 2, one of the converged beams is incident on the substantially central position of a minute pattern 7 on the information carrying member 6 to form a primary beam spot 8, while the remaining two beams are distributed over the pattern 7 in the directional components orthogonal to the tracing direction of the minute pattern 7 to form secondary beam spots 9 and 10, respectively.
The reflected light of the beam spot 8 incident on the substantially central position of the pattern 7 is optically modulated by the minute pattern 7 and, by way of the objective lens 5, half-mirror 4 and a condensing lens 11, it is photoelectrically converted by a photoelectric converter 12 such as a PIN photodiode whereby an information signal V.sub.A is generated from a current-to-voltage converter 13, such as shown in FIG. 3. On the other hand, the reflected light beams of the aforesaid remaining two converging light beams are optically modulated by the minute pattern 7 and these modulated light beams are photoelectrically converted by photoelectric converters 14 and 15, respectively. These converted signals are supplied to current-to-voltage converters 16 and 17, respectively, and then supplied to low-pass filters 18 and 19, respectively, for elimination of the frequency components of the aforesaid information signal V.sub.A. Subsequently, the output signals of the low-pass filters 18 and 19 are applied to a differential amplifier 20 to detect the difference between these signals to thereby detect a tracking error signal V.sub.B. Denoted by a chain-line 21 in FIG. 1 is an optical head, while denoted by an arrow X in FIG. 2 is the direction of movement of the minute pattern 7.
(2) Tracking Error Signal Detecting Device of Feterodyne System
FIG. 4 shows an example which is disclosed in Japanese Patent Application Laid-Open No. 3236/1982, in which a single beam is converged on an information carrying member and the light beam optically modulated by the fine pattern is directed to a four-division photoelectric converter 22. The converter 22 comprises four elements 22A, 22B, 22C and 22D constituting two pairs of diagonally opposing elements. The outputs of the elements in each pair are added to each other by an adder 23A or 23B. Then, at a subtractor 24A and an adder 24B, the subtracting and adding operations are effected relative to the outputs of the adders 23A and 23B to obtain an information reproducing signal RF and a differential signal DL. Next, by using a pulse generator 25, a pulse is generated at the zero-cross time of the information reproducing signal RF, and by using it as a sampling pulse, a sample-hold circuit 26 sample-holds the differential signal DL. The held signal after being filtered by a low-pass filter 27 is detected as a tracking error signal V.sub.C.
(3) Tracking Error Signal Detecting Device of Push-Pull System
In this device, a single light beam is converged on an information carrying member and the light beam optically modulated by the minute pattern is directed to a two-division photoelectric converter. The difference between the output of two elements of the photoelectic converter is detected as the tracking error signal while the sum of the outputs is detected as the information signal.
These conventional devices have several problems such as follows:
In the detecting device described in Item (1), the light beam emerging from the light source is divided into three beams. Therefore, the quantity of each of the divided light beams can not be increased which is a bar to realization of higher S/N ratio in generating the detection signal.
In the detecting device described in Item (2), a complicated structure is required to effect the sample-hold operation relative to the light beam particularly when the frequency of the information signal is high.
In the detecting device described in Item (3), the photoelectric converter is usually located at the position which is not optically conjugate with the beam spot on the information carrying member. Therefore, when the information carrying member is moved in the directions orthogonal to the surface thereof, the optical path is changed so that the shape of the pattern projected onto the photoelectric converter is changed. Whereby it is difficult to detect any tracking error precisely. If, in order to overcome such problem, the photoelectric converter is disposed in the optically conjugate relation with the beam spot on the information carrying member, a greater length of the optical path is required because two photoelectric converter elements of ordinary size are necessary for separately detecting the optical modulation of the tracking error.